1. Field of the Invention
The present invention relates to a direct reduction method and a rotary hearth furnace. More particularly, the present invention relates to a direct reduction method which can improve productivity and reduce the fuel consumption per unit product by effectively performing control of a heating atmosphere in a furnace, and relates to a rotary hearth furnace for carrying out the direct reduction method.
2. Description of the Related Art
Conventionally, there is known a process wherein raw materials, including a metal oxide or a mixture of a metal oxide and a carbonaceous material as reductant, e.g., coal, in the form of agglomerates, such as pellets and briquettes, are charged into a rotary hearth furnace, and the metal oxide in the raw materials undergoes direct reduction for recovery of a metal. The furnace is constructed such that heat is supplied to the raw materials from a plurality of burners in the direct reduction process. During the direct reduction process, the inside of the furnace is heated to and held at temperatures in the range of 1200-1500.degree. C.
For the technique regarding direct reduction methods using rotary hearth furnaces, there is known a method of supplying a metal oxide, which contains a reluctant, to a rotary-hearth type calcining furnace for heating and reducing the metal oxide in the furnace, as disclosed in, for example, U.S. Pat. No. 4,622,905. In the disclosed method, multitype burners are installed on a side wall, and the furnace is operated under mixed combustion with pulverized coal to enhance emissive power of flames at or near an equivalence ratio 1.0.
U.S. Pat. No. 4,701,214 discloses that air in an amount required to burn a flammable gas generated from raw materials in association with reduction is fed as surplus air to burners installed on a side wall. The publication also discloses that the aforementioned atmosphere can be realized by the use of gaseous or liquid fuel and preheated combustion air.
According to the aforementioned related art, in manufacture of a metal, such as reduced iron, using a rotary hearth furnace, air is fed into the furnace by burners, which are installed on a side wall, in an amount near an equivalence ratio 1.0 relative to a flammable gas generated with burner combustion and reduction. Furthermore, in a walking-hearth type heating furnace for continuously heating raw materials, "end burners" or "roof burners" are employed as burners installed at the ceiling.
The inventors have also conducted studies on the structure of a rotary hearth furnace for many years, and have proposed a technique disclosed in Japanese Unexamined Patent Application Publication No. 10-60514 (U.S. Pat. No. 5,989,019). In the proposed technique, means for feeding a gas for secondary combustion to burn a flammable gas generated from objects to be heated is provided near a hearth below at least one of burners provided in an upper portion of a side wall. With this construction, the flammable gas generated with reduction can be burnt at high efficiency, and hence serves as a heat supply source for raw materials. As a result, an amount of fuel fed to the burners in the rotary hearth furnace is cut down, whereby the fuel consumption per unit product is reduced.
In the process of direct reduction from a metal oxide, by charging raw materials, including the metal oxide and a carbonaceous material, into a furnace and heating them, reduction reaction progresses and the metal oxide is reduced. When the raw materials are heated to temperatures of 1000.degree. C. or higher during the process, reduction reaction rapidly progresses. Thus, in order to achieve high productivity, the raw materials require to be rapidly heated immediately after being charged into the furnace.
On the other hand, at the time the raw materials are charged into the furnace, a large amount of a flammable gas, containing CO, H.sub.2, CH.sub.4, etc., is generated. The amount of the flammable gas decreases with time and an amount of heat applied to the raw materials is changed correspondingly. More specifically, in a latter reducing period, a metal having been once reduced may be oxidized again (re-oxidized) by CO.sub.2, H.sub.2 O, etc., which are contained in a combustion exhaust gas, due to a reduction in amount of the flammable gas generated from the raw materials and having a reducing action. For this reason, a reducing atmosphere must be maintained around the raw materials in the latter reducing period.
Moreover, because the fuel consumption per unit product in the process can be improved by utilizing, as a heat source, the flammable gas generated with reduction of the metal oxide, it is also important to burn the flammable gas generated from the raw materials in the furnace.
Thus, the following points have been recognized in the past; (1) reduction reaction of a metal oxide is accelerated by rapidly heating raw materials immediately after the raw materials are charged into a furnace, (2) a reducing atmosphere must be formed in a latter reducing period to prevent re-oxidization of the raw materials, and (3) the fuel consumption per unit product can be improved by burning a flammable gas generated from the raw materials. However, construction of a rotary hearth furnace and arrangement of burners for realizing the above points have not been established in the present state of the art.
According to the above-cited U.S. Pat. Nos. 4,622,905 and 4,701,214, for example, side burners provided on a side wall are employed in a rotary hearth furnace, and end burners arranged at the ceiling are employed in a walking-hearth type heating furnace. In the process wherein an amount of heat applied to raw materials is changed with time and an amount of the flammable gas generated from the raw materials is also changed, however, an effective approach to configuration of a rotary hearth furnace and installation of burners is not yet established.